BMW 318/325/M3/525/535/M5 1989-1993 Repair Information

Intake Manifold Resonance Flap Control System


See Figure 1

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Fig. Fig. 1: Intake manifold resonance flap actuator-S38 engine

As a measure to boost torque and engine performance on the S38 engine, BMW developed a system that features electronically controlled ram air induction. The BMW ram air induction system uses the energy in the intake pulses to help draw more air into the cylinders. This a non-mechanical form of supercharging; more air is forced into the cylinders using this system than the piston alone could draw.

The intake manifold is designed with a vacuum actuated flap that divides the interior. The flap will open and close depending on engine speed and throttle position. The two positions form a long path and a short path for the air to flow before it is drawn into the intake runners for each cylinder. By changing the effective length of the intake manifold, the intake pulses of air will resonate at differing frequencies. By varying the resonant frequencies as the engine speed varies, a greater ram effect can be achieved.

For the flap to actuate, two conditions must be met. The throttle must be at full open, with the full throttle switch closed, and the engine speed must be less than 4120 rpm or greater than 6720 rpm. If these conditions are not met, the flap will not actuate. When these conditions are met, the resonance flap control unit will signal an electric vacuum switch to open allowing vacuum to operate the flap. A tolerance of 60 rpm in each direction for the engine speed switching points prevents constant motion of the flap if the engine is operated at exactly either of those speeds.

On cold engine start-up, the resonance flap control unit does a self-check. The flap will be actuated and can be watched as the linkage moves. The electric vacuum valve has battery voltage applied to one terminal and the other terminal goes to the control unit. The control unit will ground the terminal allowing the circuit to be completed.

The resonance flap control unit is located under the relays in the electronics box mounted in the engine compartment. The electric vacuum valve is located under the intake manifold next to the oil trap. The flap actuator is attached to the intake manifold at the top and has a link connecting to the flap inside the manifold.

The vacuum source for the system is a vacuum reservoir mounted below the intake runner for cylinder 5. The vacuum tank is necessary since there would not be enough manifold vacuum to operate the flap at wide throttle positions. A check valve prevents loss of vacuum from the tank while the manifold vacuum is low or when the engine is not running. The tank provides enough vacuum for 6 operations of the flap without manifold vacuum.


Resonance Flap Control Unit
  1. Start the engine and watch the linkage from the vacuum actuator. The linkage should move the flap each time the engine is started.
  3. If the flap linkage doesn't move, move the linkage by hand to check is the linkage is stiff or frozen.
  5. Check the vacuum hose for cracks or breaks.
  7. Check the actuator and the electric vacuum valve.
  9. If these checks do not indicate a problem, replace the control unit.

  1. Disconnect the vacuum hose from the actuator.
  3. Connect a hand held vacuum pump to the actuator.
  5. Apply vacuum to the actuator. Replace the actuator if found faulty.

Electric Vacuum Valve
  1. Check for battery voltage to one of the terminals on the valve when the ignition switch is ON . If there is no voltage, check the wire going to the control unit in the electronics box.
  3. If there is voltage to the terminal, ground the other terminal. This will simulate the operation of the control unit and the valve should operate.
  5. Replace the valve is found defective.